diff: don't read index when --no-index is given

git diff --no-index ... currently reads the index, during setup, when
calling gitmodules_config().  This results in worse performance when the
index is not actually needed.  This patch avoids calling
gitmodules_config() when the --no-index option is given.  The times for
executing "git diff --no-index" in the WebKit repository are improved as
follows:

Test                      HEAD~3            HEAD
------------------------------------------------------------------
4001.1: diff --no-index   0.24(0.15+0.09)   0.01(0.00+0.00) -95.8%

An additional improvement of this patch is that "git diff --no-index" no
longer breaks when the index file is corrupt, which makes it possible to
use it for investigating the broken repository.

To improve the possible usage as investigation tool for broken
repositories, setup_git_directory_gently() is also not called when the
--no-index option is given.

Also add a test to guard against future breakages, and a performance
test to show the improvements.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -298,7 +298,9 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			break;
 	}
 
-	prefix = setup_git_directory_gently(&nongit);
+	if (!no_index)
+		prefix = setup_git_directory_gently(&nongit);
+
 	/*
 	 * Treat git diff with at least one path outside of the
 	 * repo the same as if the command would have been executed
@@ -311,7 +313,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			!path_inside_repo(prefix, argv[i + 1]))))
 		no_index = DIFF_NO_INDEX_IMPLICIT;
 
-	gitmodules_config();
+	if (!no_index)
+		gitmodules_config();
 	git_config(git_diff_ui_config, NULL);
 
 	init_revisions(&rev, prefix);

t/perf/p4001-diff-no-index.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+test_description="Test diff --no-index performance"
+
+. ./perf-lib.sh
+
+test_perf_large_repo
+test_checkout_worktree
+
+file1=$(git ls-files | tail -n 2 | head -1)
+file2=$(git ls-files | tail -n 1 | head -1)
+
+test_expect_success "empty files, so they take no time to diff" "
+	echo >$file1 &&
+	echo >$file2
+"
+
+test_perf "diff --no-index" "
+	git diff --no-index $file1 $file2 >/dev/null
+"
+
+test_done

t/t4053-diff-no-index.sh

@@ -29,4 +29,19 @@ test_expect_success 'git diff --no-index relative path outside repo' '
 	)
 '
 
+test_expect_success 'git diff --no-index with broken index' '
+	(
+		cd repo &&
+		echo broken >.git/index &&
+		git diff --no-index a ../non/git/a
+	)
+'
+
+test_expect_success 'git diff outside repo with broken index' '
+	(
+		cd repo &&
+		git diff ../non/git/a ../non/git/b
+	)
+'
+
 test_done